1. Field of the Invention
The present invention relates to an optical module such as a laser diode (LD) module, for example. In particular, the invention relates to a method of adjusting the rotation angle of a polarization-maintaining optical fiber relative to a light source.
2. Description of the Prior Art
Examples of an optical module include a laser diode module provided with a laser diode, for example. A polarization-maintaining optical fiber is connected to the laser diode, for example. A polarized emission from the laser diode enters the polarization-maintaining optical fiber. The linear polarization is preserved in the polarization-maintaining optical fiber. “Extinction ratio” represents the characteristic of the polarization-maintaining properties of the polarization-maintaining optical fiber. A predetermined value of the extinction ratio must be established in the polarization-maintaining optical fiber. The fluctuation range of the extinction ratio gets wider in proportion to a drift between the polarization plane of an incident light from the laser diode and the birefringent axis of the polarization-maintaining optical fiber. The relative rotation angle must be adjusted between the laser diode and the polarization-maintaining optical fiber for obtaining the predetermined value of the extinction ratio. Moreover, the extinction ratio depends on the stress generated in the polarization-maintaining optical fiber. For example, the expansion and shrinkage or bending, resulting from a change in temperature, of the polarization-maintaining optical fiber induce a change in the extinction ratio. In addition, the extinction ratio must be restricted within a predetermined range. The range of the extinction ratio is determined based on the relative rotation angle between the laser diode and the polarization-maintaining optical fiber. The relative rotation angle is precisely adjusted around an optical axis between the laser diode and the polarization-maintaining optical fiber during the assembling of a laser diode module.
The extinction ratio is measured during the adjustment of the relative rotation angle between the laser diode and the polarization-maintaining optical fiber. As described above, since the extinction ratio depends on the stress generated in the polarization-maintaining optical fiber, a load is applied to the polarization-maintaining optical fiber during the measurement of the extinction ratio. The stress changes in the polarization-maintaining optical fiber based on the magnitude of the load. The change in the stress induces change in the extinction ratio. The extinction ratio is measured at various values of the load for grasping the change in the extinction ratio resulting from the change in the stress. Moreover, since the extinction ratio depends on the relative rotation angle between the laser diode and the polarization-maintaining optical fiber, the measurement of the extinction ratio must be conducted at a plurality of relative rotation angles. The relative rotation angle is forced to shift by 0.1 degrees over a range of 20 degrees around the optical axis between the light source and the polarization-maintaining optical fiber in the measurement of the extinction ratio, for example. Specifically, the extinction ratio must be measured at every 0.1 degree. As a result, much time and labor are required to adjust the polarization-maintaining optical fiber relative to the light source around the optical axis.